1. Field of the Invention
This invention relates to a radiation image read-out apparatus for exposing a stimulable phosphor carrying a radiation image stored therein to stimulating rays which cause the stimulable phosphor to emit light in proportion to the stored radiation energy, and detecting the emitted light for obtaining an image signal. This invention particularly relates to a radiation image read-out apparatus for conducting image read-out by a photodetector using a photoelectric conversion device fabricated of Si or the like.
2. Description of the Prior Art
A novel radiation image recording and reproducing system is disclosed, for example, in U.S. Pat. No. 3,859,527. The system comprises (i) exposing a stimulable phosphor sheet to a radiation passing through an object such as the human body to have a radiation image stored therein, (ii) scanning the stimulable phosphor sheet by stimulating rays which cause it to emit light in proportion to the radiation energy stored, (iii) detecting the emitted light and converting it into an electric image signal by use of a photodetector, and (iv) reproducing a visible image by use of the electric image signal.
In the aforesaid radiation image recording and reproducing system, during image read-out, a semi-transparent mirror is positioned at an angle of 45.degree. with respect to the stimulable phosphor sheet, and stimulating rays are made to pass through the semi-transparent mirror from the back thereof and to impinge upon the stimulable phosphor sheet. Light emitted by the stimulable phosphor sheet in proportion to the stored radiation energy when it is exposed to stimulating rays is reflected approximately normal to the optical path of the stimulating rays by the semi-transparent mirror and is received by an image intensifier tube or a photomultiplier. Alternatively, stimulating rays are emitted from the rear surface of the stimulable phosphor sheet via an aperture, and light emitted by the front surface of the stimulable phosphor sheet is laterally reflected by a prism and received by an image intensifier tube. However, since the semi-transparent mirror or the prism is positioned in far spaced relation to the stimulable phosphor sheet, it is not always possible to efficiently guide the light emitted by the stimulable phosphor sheet, which is non-directional and weak.
Also, it has been proposed in U.S. Pat. No. 4,302,671 to position a photodetector close to a stimulable phosphor sheet, and to position a small reflecting optical element between the photodetector and the stimulable phosphor sheet so that stimulating rays passing through the space between the photodetector and the stimulable phosphor sheet are reflected by the reflecting optical element and made to impinge upon the stimulable phosphor sheet. In this technique, since the light receiving solid angle of the photodetector can be made large, it is possible to improve the signal-to-noise ratio. However, it is necessary to use a complicated, precise optical device for guiding the stimulating rays through the space between the stimulable phosphor sheet and the photodetector to the small reflecting optical element.
In order to eliminate the aforesaid problems of the conventional techniques, a photodetector comprising many photoelectric conversion devices fabricated of Si or the like, for example, a line sensor or a matrix type sensor standing face to face with the whole surface of the stimulable phosphor sheet, may be positioned close to the surface of the stimulable phosphor sheet, and the output of the photodetector may be detected by an image read-out means. In this case, it becomes possible to efficiently detect weak light emitted by the stimulable phosphor sheet, to shorten the read-out time, and to make the read-out apparatus small.
However, the aforesaid photoelectric conversion devices exhibit sensitivity only to a specific wavelength range. Therefore, when the major wavelength distribution of the light emitted by the stimulable phosphor sheet is outside of the sensitive wavelength range of the photoelectric conversion devices, the sensitivity of the photodetector becomes insufficient. For example, BaFBr:Eu, BaClBr:Eu, or the like is preferably used as the stimulable phosphor, and Si devices exhibiting small dark current, high quantum efficiency and easy to fabricate are generally used as the photoelectric conversion devices of the photodetector. The wavelength distribution of the light emitted by the preferable stimulable phosphor such as BaFBr:Eu or BaClBr:Eu is mainly from 350 nm to 420 nm, and the major sensitive wavelength range of the Si elements is higher than 440 nm. Therefore, in this case, the photodetector cannot sufficiently detect the light emitted by the stimulable phosphor.